Zika virus (ZIKV) spread explosively throughout the Americas in 2015, leaving in its wake a devastating and ever-expanding list of sequela associated with infection. Since this epidemic, substantial effort has been put forth to understand the emergence of ZIKV in the Americas, the course of disease and importantly, the correlates of protection. ZIKV is a member of the flavivirus genus which includes other arthropod-borne human pathogens such as yellow fever virus (YFV), West Nile virus (WNV), and the four serotypes of dengue virus (DENV). In addition to circulating in mosquito populations in the same geographic regions, flaviviruses share a substantial degree of genetic, and consequently antigenic similarity. This begs the question: In areas of endemic flavivirus circulation, how does immunity to one flavivirus shape immunity to the next? Moreover, how does this divergent immune restriction alter the viral swarms that replicate within the host? These questions are of great importance due to what we know about DENV. The enhanced disease seen in secondary infection with a heterologous serotype of DENV has been hypothesized to be mediated by both suboptimal cross-reactive antibody and T cell responses. The proposed research study is to define how exposure to a heterologous flavivirus shapes the transcriptional and functional T cell response to ZIKV in a mouse model of infection, and how those T cells in turn impact viral populations within the host and ultimately pathogenesis. We have shown previously that CD4+ and CD8+ T cells play important roles in protection from ZIKV disease in a mouse model of infection and were able to identify specific epitopes to which the responses are directed in this H2b-restricted model. We have also generated data showing that effector CD8+ T cells generated during infection with DENV1-4, YFV, Usutu virus, Kunjin virus or WNV functionally cross-react with at least one ZIKV CD8+ epitope, which we are terming a ?pan-flavivirus? epitope. We have found that prior infection with a heterologous flavivirus results in altered phenotypic responses to this cross-reactive epitope during a subsequent ZIKV infection compared to a homologous ZIKV prime-boost challenge or a primary ZIKV challenge. One specific example of this is that cross-reactive cells derived from a heterologous infection produce far more Granzyme B relative to homologously derived cells. However, it is still unknown what drives these functional changes and how they impact protection from ZIKV-induced disease, which will be evaluated in Aim 1. More recently, it is being appreciated that different immune selective pressures can impact the viral populations that evolve within the host during infection, particularly in RNA viruses. How this observed altered T cell function during heterologous ZIKV infection alters viral quasispecies diversity within the host will be evaluated in Aim 2.